Introduce Capability Registry & Discovery

tests/Capability/Discovery/Fixtures/InvocableResourceFixture.php

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+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Tests\Capability\Discovery\Fixtures;
+
+use Mcp\Capability\Attribute\McpResource;
+
+#[McpResource(uri: 'invokable://config/status', name: 'invokable_app_status')]
+class InvocableResourceFixture
+{
+    public function __invoke(): array
+    {
+        return ['status' => 'OK', 'load' => rand(1, 100) / 100.0];
+    }
+}